The low pressure fuel system of a typical heavy duty diesel engine having a common rail fuel system consists of a transfer pump which lifts fuel from a fuel tank via a first stage, coarse mesh fuel filter. The transfer pump raises the pressure of the fuel to an intermediate pressure level and the fuel is then passed to the inlet of a high pressure fuel pump via fine mesh filters. In order that the fuel pressure at the output of the high pressure pump can be closely controlled, it is necessary to control the pressure of fuel at its inlet. It is known to use a fuel pressure regulation system in which a fuel pressure regulator spills fuel from the high pressure side to the low pressure side of the transfer pump, to maintain the fuel pressure at the inlet to the high pressure pump at or below a pre-determined level. In prior art arrangements the spill line is located downstream of the fine mesh filters. As a result the flow rate through the fine mesh filters is relatively high, because all of the fuel pumped by the transfer pump passes through them. Consequently, the working life of the filters is reduced because the pressure drop across the filters increases as the flow rate increases and hence the maximum allowable pressure drop across the filters is reached more quickly with high flow rates.